Google's Jessica said they removed it to "very low usage" and said removing it will streamline searches. She explained in the Google Web Search Help forums:

As we've streamlined the results page, we've had to remove certain features, such as Instant Previews.

Instant previews saw very low usage by our users, and we've decided to focus on streamlining the page to benefit more users.

Like with any feature Google removes, it upsets users. So there are a bunch of people not happy.

With this change, Google has changed how you access the "cached" page and how you share results. There is now a down green arrow that contains the "cached" link and "similar" link, when applicable and if you are logged in, it will also show a "share" link to share the result on Google+.